1. Field of the Invention
The present invention relates to an interrupt control system provided in a computer, and more particularly to an interrupt control system for controlling the interrupt signals from PCI devices and ISA devices.
2. Description of the Related Art
ISA (Industry Standard Architecture) buses and EISA (Extended Industry Standard Architecture) buses have been widely used as system buses used for personal computers. Recently, however, PCI (Peripheral Component Interconnect) buses have come into use, centering on desktop personal computers, in order to step up the data transfer speed or construct a system architecture independent of processors.
With a PCI bus, almost all data transfer is based on block transfer. Each block transfer is realized by burst transfer, which enables data to be transferred at a speed of up to 133 Mbytes per second on a PCI bus (in the case of a 32-bit data bus).
Therefore, use of PCI buses makes it possible to speed up the data transfer between I/O devices and between the system memory and I/O devices, enhancing the system performance.
Recently, not only desktop personal computers but also notebook or desktop portable personal computers have employed PCI buses.
In a desktop personal computer, optional cards for functional expansion such as PCI expansion cards or ISA expansion cards are installed directly in the expansion slots. In this case, a plurality of interrupt request signal lines differing in the interrupt level (the order of priority of interrupt) are defined for the individual ISA expansion slots to which ISA expansion cards are connected. This enables each ISA expansion card to inform the interrupt controller on the system board of an interrupt request via the interrupt request signal line corresponding to a particular interrupt level given to the card.
However, the interrupt request signal lines (INTA#, INTB#, INTC#, INTD#) used by the PCI devices and PCI expansion cards on the system board differ from the interrupt request signal lines (IRQ0 to IRQ15) used by the ISA devices and ISA expansion cards in the interrupt request signal sensing type and the polarity. Specifically, INTA# to INTD# are of the level sensing type and low active signals, whereas IRQ0 to IRQ15 are of the edge sensing type and high active signals.
Therefore, a system employing a PCI bus must handle two types of interrupt request signals differing in features, with the result that an interrupt controller compatible with 82C59, the standard in the ISA system, cannot be used for a PCI system without any modification.
Accordingly, it is necessary to improve the interrupt controller so that the two types of interrupt requests of IRQ0 to IRQ15 and INTA# to INTD# may be handled.
The portable personal computer has the following problem because it processes the interrupt signal from an ISA expansion card.
Specifically, in a portable personal computer, optional cards, such as PCI expansion cards or ISA expansion cards, are installed via expansion units known as docking stations. In this case, the number of lines drawn into docking station from the portable computer is required to be as small as possible. The reason for this is that an increase in the number of signal lines drawn into the docking station leads to an increase in the number of pins of an LSI interfacing with the docking station mounted on the system board and an increase in the number of pins of a connector connecting the portable personal computer with the docking station. This raises the cost per portable personal computer and lowers the system packaging efficiency.
Therefore, it is undesirable to draw a plurality of interrupt request signal lines (corresponding to IRQ0 to IRQ15) into a docking station. Unless these interrupt request signal lines are drawn into the docking station, such optional cards as ISA expansion cards cannot receive interrupt service of the CPU.
To overcome this problem, there has recently appeared a protocol for enabling an ISA expansion card installed in the docking station to inform the interrupt controller on the system board of an interrupt request signal.
With the protocol, a single exclusive serial data line is used to transfer a plurality of interrupt requests serially.
When the serial interrupt protocol that enables a plurality of interrupt levels to be transferred serially is used, both of PCI expansion cards and ISA expansion cards can be used just by drawing the signal lines corresponding to the PCI bus and the data lines for serial transfer into the docking station and providing a PCI-ISA bridge in the docking station.
Use of such a serial interrupt protocol, however, requires the interrupt controller to be modified so as to be able to deal with not only the aforementioned two types of interrupt requests of IRQ0 to IRQ15 and INTA# to INTD# but also serial interrupts.